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Optical direct-reading particle counting techniques have the advantage of rapid, continuous, nondestructive particle detection. However, the amount of light scattered may not be directly related to the property that one wishes to measure. By combining the advantages of optical detection techniques with the manipulation of particle motion, several instruments have been developed that detect more specific properties of aerosol particles. The aerodynamic size of particles is used to describe the behavior of particles in gravitational settling, filtration, respiratory deposition, sampling systems, etc. Measurement of the aerodynamic size at one time could only be achieved by manually observing the settling velocity of individual particles. Subsequently, impactors allowed the measurement of size distributions on a routine basis, although gravimetric and/or chemical analysis still had to be carried out in the laboratory. With the advent of new technology (e.g., lasers and microcomputers), real-time measurements became possible. Several instruments were developed to measure the aerodynamic size as rapidly and accurately as possible. These included the single-particle aerodynamic relaxation time analyzer (E-SPART; Hosakawa Micron International, Osaka, Japan; distributed by Micron Powder Systems, Summit, NJ), the Aerodynamic Particle Sizer (APS; TSI, Inc., St. Paul, MN) and the Aerosizer (Amherst Process Instruments, Hadley, MA). While the latter two instruments allow a rapid determination of size distributions, they measure particle behavior largely outside the Stokes regime and the recorded size must be corrected to give an accurate aerodynamic size of individual particles. While the aerodynamic diameter describes the inertial properties, the electrostatic charge influences the electrodynamic behavior of the particle in transport processes. Both aerodynamic diameter and electrostatic charge measurements on individual particles are needed in many electrodynamic processes; some examples are electrophotography and laser printing, electrostatic powder coating, electrostatic precipitation, electrostatically enhanced fabric filtration, and electrostatic benefication of minerals and coal. The E-SPART is capable of measuring particle charge as well as aerodynamic diameter. Airborne abestos fiber measurements went through a similar progression over time in that, originally, relative crude measurements of concentration were made by collection with midget impingers and microscope counting of all large particles. Filter collection with microscopic analysis was developed so that only fibers were detected. Finally, the development of the fibrous aerosol monitor (model FAM-1 MIE Inc., Bedford, MA) allowed continuous, real-time detection of airborne fibers. The FAM-1 was designed to give results close to those of the phase contrast light microscope method (see Chapter 25). These more sophisticated instruments provide more specific data about aerosols; however, because of the complexity of their detection and analysis systems, they may also have various limitations and subtle problems associated with the interpretation of the data. The following sections present a discussion of these instruments.